[sdiy] Oberheim TVS (original) digital keyboard module design

[Travis Thatcher]

Recently I acquired a TVS in pretty remarkable physical condition and
mostly fully functional electronically. When I got it everything
worked except the keyboard did not actually play the correct notes. It
did properly trigger on all notes. I added sockets for all the ICs in
the keyboard module as its a known point of failure. After a few
mishaps with some lifted traces burning out a 4016 and some other
screw ups I have it 99% there.

The last 4 notes are the issue now as they are triggering but again
playing the wrong pitches. I assume I borked up either the flip-flops,
the 4512 or the 4028. I'm ordering new chips for those now. I did
notice that if I use a different 4512 it still plays wrong pitches for
the last 4 notes but differently wrong pitches.

My question is that of the design of the actual module. I sort of
understand the addressing and general gist of it but I'm not entirely
sure how the dac works here. The logic for the gates seems to make
sense but from an educational standpoint I'd love it if someone
familiar with these circuits could help me get a better understanding.
Also - the parts I am using are CD CMOS series, CD4512BE, CD4028BE and
CD40174BE. I'm specifically curious if the CD40174BE is a good
replacement for the 74C174s that were originally there. I know all the
supplies for the chips in this circuit are +9v rather that 5v TTL
logic i'm used to with this stuff.

Schematic can be found here:
http://www.ersatzplanet.com/Pages/pdf/ObieSchemos/2V_Digital_Keyboard.pdf

Thanks for any info - I'm really excited to have this thing fully up
and running and I'd love to have a better understanding of the
circuitry behind it.

Travis

-- 
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